You can get more than 10 Play Coins a day by changing the date under the system settings. You can earn one Play Coin by taking 100 steps with the 3DS in Sleep mode. Two Play Coins can buy either a Wandering Hero (Find Mii) or Puzzle Piece (Puzzle Swap).

Puzzle Swap

You must complete two separate puzzles by collecting pieces that other people have found in StreetPass adventures. You can get a puzzle piece from another Mii's puzzles to complete your own.

Personal greeting

StreetPass with the same person multiple times to get a personal greeting.

Miis

Each Mii collected through StreetPass becomes a "unit" that can be used in the dungeons. Each attack and progress made will carry over to the next time you enter Find Mii. If you StreetPass with the same person multiple times, that person's Mii will level up and become more powerful. The shirt color of your collected Mii units corresponds to the type of magic they use.